1. Field of the Invention
The present invention relates to a drive circuit for a display driven by an alternating current (AC), more particularly relates to a drive circuit for an electroluminescence (EL) display device and a liquid crystal display device including a plurality of passive addressing type and active addressing type liquid crystal pixels arranged in a matrix form, further particularly relates to a construction of an integrated circuit for driving a common timing electrode and a construction of a drive circuit using the same.
2. Description of the Related Art
There are many electrical components and devices which are driven by AC power. The voltage necessary to drive those is obtained by transforming commercial voltage utilizing a transformer.
When the frequency of the necessary voltage differs from the commercial voltage, the necessary voltage has been obtained by first preparing and amplifying an AC drive waveform from a direct current (DC) voltage utilizing a semiconductor device and then adjusting the voltage level by adjusting the DC voltage, wave form, or winding ratio of the transformer.
On the other hand, in order to actually obtain a display driving wave voltage generating circuit having a small volume and light weight, the waveform and the voltage can be controlled by a circuit using semiconductor devices.
In this case, the DC voltage of the voltage generating circuit must be more than .sqroot.2 times the effective value of the driving AC voltage required. In order to obtain a high AC voltage, there is the push-pull drive method.
Two voltage generating circuits having AC amplitudes of opposite polarity are prepared, and a drive voltage of a maximum two times that of the source voltage can be generated by driving the devices utilizing the difference between the two voltages.
In this circuit, a DC voltage component in the push-pull drive method can be canceled out by utilizing two voltage waveforms having the same polarity and the same amplitude.
When a plurality of devices are intended to be driven, the devices are arranged in a matrix form and driven utilizing the push-pull drive principle.
Note, that in this system, the devices are classified into several groups.
One end of a device belonging to one group is driven by a timing signal with a waveform defined by a function of a time a constant period.
Another end is driven by a drive waveform of the opposite or same polarity as the timing signal in response to whether the drive for the devices is ON or OFF.
One example of such a driving method is disclosed in the Journal of the Society of Information Display, Vol. 26/1, 1985, page 9 to 15.
According to this drive method, an AC voltage having higher voltage level than the DC voltage can be applied across the terminals of a device, although a semiconductor switching device having a high withstand voltage is required to drive high voltage drive devices.
On the other hand, a matrix type drive system is usually used for a displaying apparatus because the numbers of displaying pixels thereof are generally large.
An integrated circuit consisting of transistors and having multiple output terminals can be used for driving liquid crystal or EL type display means.
However, when high integration density is required, the display means must be driven with a low voltage; when a high withstand voltage must be realized, a low integration density is required; and when a high processing speed should be realized, the display means must be driven with a low voltage.
It is very difficult to make an integrated circuit satisfying all of the above requirements simultaneously.
Generally, in designing a complementary field effect transistor integrated circuit (C/MOS-IC), the source voltage is set at 5 V.
When designing for a source voltage of more than 5 V, the integration density of the circuit remarkably falls and the operating speed of the IC declines.
For example, when the IC is driven by 5 V, the response speed is about 50 MHz, while when driven by 25 V, the response speed is about 5 MHz and the integration density is 1/4 of the former one.
The insufficient operating speed resulted in it being thought difficult to obtain a fine image display in a liquid crystal display device.
One method for solving this is to design those parts of an IC requiring a quick response to operate at 5 V, to add a logic level converting circuit that greatly amplifies the logic amplitude, and to connect parts driven by a large amplitude to the IC circuit in a high withstand voltage design to satisfy the dual requirements of high speed operation and high withstand voltage.
The construction above, however, requires provision in the IC for many level shifters, which , in turn, require either enlarging the IC chip or reducing the functions included in one IC chip, thus resulting in an extremely uneconomical IC.
The present inventor has already proposed an idea for improving this in Japanese Unexamined Patent Publication (Kokai) No. 60-249191.
According to the proposal, a pulse signal having a differential voltage exceeding the source voltage can be obtained by adding up a first pulse signal generated from a pulse generating circuit, and a second pulse signal having a different voltage level obtained from the first pulse signal, utilizing a clamping circuit.
There are limitations in usage of this method, however, since the pulse signals which can be used are restricted to those where the low voltage level of the first pulse signal and the high voltage level of the second pulse signal are close to each other.
The object of the present invention is to overcome the drawbacks in the conventional circuits and to provide a circuit which utilizes a semiconductor IC having relatively low withstand voltage to produce a drive waveform of a regular high voltage which exceeds the withstand voltage.